Hundreds of millions of tons of discarded coal litter the landscape in coal producing regions throughout the United States. Discarded coal includes run-of-mine coal and previously processed or waste coal. Run of mine coal is coal discharged from mining operations prior to processing. It is produced from both underground mining operations and open pit mining or surface mining operations. Run-of-mine coal often contains unwanted impurities such as rock, minerals, and dirt, and comes in a mixture of different-sized fragments.
Waste coal is sometimes referred to as “gob” (garbage of bituminous) or “boney” in the bituminous coal mining regions of western Pennsylvania and West Virginia. Millions of tons of waste coal are deposited in coal ponds and gob piles in these mining regions, as well as in the Appalachian coal region including Virginia, Kentucky, and Alabama, as well as in the Midwest, (e.g., the Illinois basin, Indiana, etc.). Coal ponds and gob piles include coal that has been previously mined and processed, and contain coal of various qualities and sizes. Gob piles and coal ponds often contain coal particle that are small in size and that have high ash content (i.e., non-coal minerals and impurities). This is because, in the past, coal use often focused on larger particle sizes.
Moreover, because of limited knowledge and technology, run-of-mine coal below about one-quarter of an inch (¼″) in size typically was discarded. In addition, various industries typically required coal having a particular ash content. The coal not complying with the required ash content after beneficiation processing was discarded. For example, coal processed for high grade markets, such as the steel industry, was often washed to provide a low ash content (e.g., about 4% ash). Coal utilized in electricity generating power plants (“thermal coal”) was often washed so as to have an ash content of 10% to 15%.
Discarded coal typically has a lower energy value than pure coal. However, because of increasing energy demands and because of environmental concerns, it has become desirable to remove and reclaim discarded coal from gob piles, ponds, and other disposal locations.
The processing of coal to remove impurities for both the thermal, metallurgical and petroleum coal markets has historically been accomplished via wet processes involving crushing and float/sink separation based on specific gravity. Current practices for cleaning coal involve crushing to liberate impurities and submitting various size fractions to float/sink devices and circuits to isolate coal from the impurities such as silica, clay, and sulfur. For example, after coal is crushed, it is separated into various-sized fragments. The coal can then be separated from other impurities, such as rock, by being floated in a tank containing a high-density liquid. Because the coal is lighter, it floats and is separated off, while heavier rock and other impurities sink and are removed as waste. Unfortunately, because discarded coal may contain as much as 50% ash, conventional ash-removal techniques may not be effective or efficient in reclaiming discarded coal. The larger a coal particle, the more likely that the coal particle will have impurities entrained therein. In conventional float/sink coal processing components, a given coal particle will report based upon the average of the specific gravity or density of the particle. For example, a lump crushed to two inches (2″) may contain about 50% “pure” coal at a specific gravity of about 1.3, about 46% silica and clay at a specific gravity of about 2.6, and about 4% sulfur at a specific gravity of about 5.0. Therefore, the coal particle exposed in a wet float sink device or circuit will report as a particle having a specific gravity of about 2.0 (the weighted average of the individual mineral components). Coal float sink processes generally function in a range from 1.30 to 1.80 specific gravity separations. As such, the pure coal in significant quantity would sink as a reject and would be discarded.